1. Field of the Invention
The present invention relates to an encased transformer for use in audio equipment, video equipment, and any other electrical equipment.
2. Description of the Prior Art
In order to explain the background of the present invention, reference will be made to FIGS. 1 to 7, which illustrate a typical conventional transformer:
As shown in FIGS. 1 and 2, the transformer includes a bobbin 4 having a cylindrical portion 1 with an upper collar 2 and a lower collar 3 at its upper and lower ends, respectively. The lower collar 3 is provided with outwardly laterally oriented metal plate terminals 5a to 5d which are formed in one piece with the lower collar 3 or alternatively attached thereto after they are made by molding. Lead wires 6a to 6d of primary and secondary coils 7, 8 wound round the bobbin 4 are bent at their lower portions at a desired angle, with portions of several millimeters left for the convenience of the connection to the metal plate terminals 5a to 5d as used by the user (hereinafter referred to as "user metal terminal"), and the top ends of the lead wires 6a to 6d are outwardly bent at a desired angle so that they are positioned at the same level with the underside 3' of the lower collar 3. The primary coil 7 is wound round the cylindrical portion 1 of the bobbin 4 of the above-described construction, a lead wire 6a at the leading end thereof being passed through a groove 9a provided in the lower collar 3 for winding round the base 5e of the user metal terminal 5a. A lead wire 6b at the trailing end of the coil is passed through a groove 9b for winding round the base 5f of the user metal terminal 5b. The secondary coil 8 is wound round the primary coil 7, a lead wire 6c at the leading end thereof being passed through a groove 9c provided in a lower collar 3b for winding round to the base 5g of the user metal terminal 5c. A lead wire at the trailing end is connection in similar manner. The lead wire 6 wound about the user metal terminal 5 are then soldered, thereafter an E-type magnetic element 10 is incorporated vertically into the cylindrical portion 1 of the bobbin 4 and fixed in position to form a magnetic path. This is a typical conventional method for constructing transformers.
Referring to FIG. 3, the construction of a conventional encased transformer will be described:
The encased transformer also includes a cylindrical portion 1 of a bobbin 4 with an upper collar 2 and a lower collar 3 provided respectively at its upper and lowed ends. User's metal terminals 5 are implanted in the lower collar 3. The cylindrical portion 1 of the bobbin 4 have primary and secondary coils 7, 8 wound round it, and lead wires 6 of the coils 7, 8 are connected to the metal terminals 5 in corresponding relation thereto by being wound about the latter. A magnetic element 10, such as a ferrite core, is vertically inserted and fixed in position to form a magnetic path. A transformer constructed in this way is inserted into an opening of a case 11, and resin material 12, such as silicon resin, is then poured into the case. Finally, a bottom plate 13 is fitted in the opening of the case 11. The amount of the resin material 12 is adjusted to cover a substantial portion of the magnetic element 10 of the transformer, and the resin is normally set by heating.
A conventional type of encased transformer used for flat mount will be described with reference to FIGS. 4 and 5.
FIG. 4 is a sectional view of the transformer. A cylindrical portion 1 of a bobbin 4 has an upper collar 2 and a lower collar 3 formed respectively at opposite ends thereof, the lower collar 3 having metal terminals 5 inserted. A primary coil 7 and a secondary coil 8 are wound round the cylindrical portion 1 of the bobbin 4, lead wires 6 of the coils being wound around metal terminals 14 (hereinafter referred to as "maker metal terminals"). A magnetic element 10, such as a ferrite core, is inserted and fixed in position. The assembly formed in this way is placed in the case 11 and then a bottom plate 13 is inserted in position to close the opening of the case 11. The opening of the case 11 is provided with a rib 15 extending along the inner circumference of the opening, and the bottom plate 13 has a stepped portion 16 extending along the outer circumference thereof. In assembling the transformer the bottom plate 13 is forced into the opening of the case 11, and after the stepped portion 16 of the bottom plate 13 is located inside of the rib 15, the expanded opening is allowed to narrow, thereby securing the joint between the bottom plate 13 and the case 11.
When an electric circuit is to be formed by using a transformer of the above described type, it is a common practice to employ a printed board. Such a circuit of the prior art will be described with reference to FIG. 6. A printed board 17 has a circuit pattern 18 printed with copper foil or the like. A transformer 19 includes various "passive elements 20", such as inductance L, capacitor C and resistance L. Active elements 21, such as transistors, are mounted in position on the printed board 17. These components are electrically connected by soldering to the printed board 17. In this way, it has been usual to have electronic parts mounted peripherally of the transformer 19 on the printed board 17. FIG. 7 is a circuit diagram showing an embodiment of FIG. 6.
Transformers of this type find many applications. For example, they are employed in electronic apparatus, such as video cameras, telephone sets, and liquid crystal TV sets. In these applications, the transformers are required to have a structure adapted for attachment on the surfaces of printed boards because of the trend toward sophisticated functions and increased versatilities of light weight and small size. On the other hand, the conditions under which transformers are used are becoming more severe than ever. For example, they are subjected to thermal stresses due to reflow soldering, and mechanical loads exerted by supersonic wave cleaning after soldering. These external stresses may break the lead wires of the transformer and/or crack the ferrite core, thus eventually breaking the transformer. This problem will be more particularly described with reference to FIG. 2:
The metal plate terminals 5a, 5b of the transformer are attached to the printed board 17. When any load is accidentally applied to the transformer in the vertical directions 22a and 22b, the transformer is actually subjected to a lift upward, that is, in the vertical direction 22a because the base portions thereof are secured to the printed board 17. In contrast, the bases 5e, 5f, 5g, 5h of the metal terminals 5a, 5b, 5c, 5d around which lead wires 6a, 6b, 6c, 6d are wound are subjected to a downward pressure together with the printed board 17 in the downward direction 22b. In this way, the lead wire 6a, 6b, are pulled both upward and downward, thereby causing the lead wires 6a and 6d to break. Likewise, when any load is applied laterally in the directions 23a and 23b, the breakage of the lead wires 6a to 6 d to occur.
Another problem is that the metal terminals 5 are easy to deform, even by a relatively light load applied to their front ends, thereby decreasing the dimensional precision. This problem derives from the fact that the entire length of each of the metal terminals 5a, 5b, is the sum of the length of each of the portions 5a, 5f, about which the lead wires 6 are wound, and the length of each of the bent potions.
A further problem derives from the reflow soldering. More specifically, when the resin cast transformer shown in FIG. 3 is to be reflow-soldered, the components are subjected to thermal stress. The problem arises during the stage of reflow soldering, in which the temperature within the reflow furnace reaches 230.degree. C. As is common with the reflow soldering process, preheating is carried out at 150.degree. C. for one to two minutes, and then the temperature of the furnace is maintained at more than 200.degree. C. for about 30 seconds. The components of the transformers are subjected to such a high temperature treatment, which unfavorably affects the transformers. For example, the primary and secondary coils 7, 8, the magnetic element 10, and the casting resin material 12 are elongated or expanded. However, since they have different coefficients of thermal expansion, the degree of expansion of the magnetic element 10, such as ferrite core, is different from that of the casting resin material 12. Generally, the casting resin material 12 has a hardness of 50 or more, and a greater expandability. Therefore, the problem is that the casting resin material 12 positioned between the bobbin 4 and the magnetic element 10, such as ferrite core, is likely to expand outwardly so that the expanding force breaks the magnetic element 10.
In a transformer of the type shown in FIG. 4, the bottom plate 13 is merely inserted in the case 11, thereby failing to achieve a strong joint therebetween. In addition, gaps are likely to be produced between the bottom of the metal terminals 5 and the bottom of the case 11 and/or the bottom of the bottom plate 13. It may happen that the metal terminals 5 are detrimentally raised relative to the bottom of the transformer. As shown in FIG. 5, the transformer having such a deficiency is mounted on the printed board 17, a gap T is created between the printed board 17 and the metal terminals 5 of the transformer, which prevents an improper soldering connection. The improper connection results in electrical conduction.
When circuits are to be constructed with the above-mentioned conventional transformer, the common practice is to mount the transformer 19 and other components on the printed board 17 as shown in FIG. 6. This practice has a drawback in that a space sufficient for mounting other components than the body of the transformer 19 is required, thereby preventing the achievement of compact-size apparatus.